Effect of Change in Ratio of Electrode to TotalPitch Length in EWOD Based Microfluidic System

Abhilash PaneriDepartment of Mechanical Engineering

Birla Institute of Technology and Science

Pilani, India 333031

Email: abhilashpaneri@gmail.com

N.N.SharmaDepartment of Mechanical Engineering

Birla Institute of Technology and Science

Pilani, India 333031

Email: nitinipun@netscape.net

Abstract—The present work investigates the effect of variationof length of electrode for a EWOD (electrowetting on dielectric)based microfluidic flow. The discussed model takes into accountthe effect of energy gradient on droplet movement. The voltageand pitch length are kept constant at 60 V and 200μm respec-tively. The parameter measured is the velocity of the droplet inthe direction along channel length.

Various configurations were simulated and the best config-uration was attributed to the one with the highest and stableenergy gradient. Simulations of droplet movement are done onCoventorWare. The increase in length of electrode has been foundto increase the velocity of droplet movement.

Index Terms—Microfluidics, Electrowetting on Dielectric(EWOD), Surface tension. Energy gradient

I. INTRODUCTION

The consequences of changing the thermal or electrical

energy in changing the surface tension, which eventually in-

duces fluid flow, were studied first time by Lippmann [3]. The

actuation of fluid flow in micro-sized channels by changing

the surface tension is an attractive option [4] and is gain-

ing importance in the developing technology of MEMS and

microfluidics. Electro capillary-based microfluidic actuation

schemes, in which the surface tension is modified by appli-

cation of an electrical potential, provide greater driving force

and consume less power than thermocapillary-based methods.

Electrocapillary effects have been used in variety of microflu-

idic actuation schemes, including continuous electrowetting

(CEW), electrowetting (EW) and Electro-wetting on dielectric

(EWOD)[1]. In CEW, a liquid metal flows continuously in

the channel containing an electrolyte under the influence of

the electric potential applied across the channel. The problem

with CEW is that it requires use of two liquids, which may

create fabrication complexities and may damage the sample.

In the EW scheme, the liquid movement in a channel is a

result of change in the surface tension of the liquid induced

by Electrical Double Layer (EDL) formed at the interface of

the channel, air, and the liquid when a voltage is applied to

the electrode. But, as the droplet is in direct contact with the

electrodes, electrolysis of water may occur at high actuating

voltages, which limits the maximum attainable actuating force

[7], unless some form of insulating layer is coated on the

electrodes to lessen the electrolytic effect.

Droplet flows in EWOD based microfluidic flow are dif-

ferent from continuous systems as they deal with individual

droplets instead of continuous liquid flow. Electrowetting-on-

dielectric (EWD) microfluidics is based on the actuation of

droplet volumes up to several microliters using the principle

of modulating the interfacial tension between a liquid and an

electrode coated with a dielectric layer [8]. In order to move

the droplet, electric field is applied to only one portion of the

droplet by establishing an electric field in the dielectric layer.

This creates an imbalance of interfacial tension, which forces

the droplet to move [9]. Droplets are usually sandwiched

between two parallel plates; the bottom being the chip surface,

which houses the electrode array and the top surface being

either a continuous ground plate or passive top plate.

EWOD based microfluidic flows are better in monitoring

the water flow and actuation of liquid flow. Because of which,

EWOD is applied to fields like drug delivery systems, medical

devices and diagnostics[11], etc. Droplet velocity in micro

channels involving EWOD based microfluidic flows are mainly

dependent upon the frequency with which the electrodes are

switched in the device[10]. More is the frequency of switching

electrodes, higher will be the velocity with which droplet under

actuation will travel.

The performance of an EWOD based microfluidic flow

depends on the size, shape and orientation of its electrodes.

Scientists around the world have tried many different shapes,

but the most extensively applied are square shaped electrodes

[1].

In recent times, developments in μ-TAS (micro total anal-

ysis systems), biosensors [2] and LOC (Lab on chip) devices,

the implementation of EWOD based microchannels is gaining

importance . In the present work, an EWOD based microfluidic

flow has been simulated for a constant potential difference to

its electrodes of 60V [6] and then keeping the pitch (electrode-

gap pair) length at constant value of 200 μm. However, the

length of the electrode is varied and corresponding droplet

velocity is observed. The work is organized in 5 sections. In

the next section, model of EWOD based microfluidic flow is

developed. Third section presents the simulation including the

boundary condition, material used and the numerical results

obtained. Fourth section discusses the results and the final

section draws conclusions based on the simulation results.

2010 International Conference on Computer Applications and Industrial Electronics (ICCAIE 2010), December 5-7, 2010, Kuala Lumpur, Malaysia

978-1-4244-9055-4/10/$26.00 ©2010 IEEE 25

II. ENERGY BASED MODEL FOR ELECTROWETTING

INDUCED FLOWS

Jones [5] developed and discussed the energy-based model

for EWOD systems for vertical flow, according to which the

reason for the movement of the droplet is energy gradient. To

actuate droplets, the interfacial energy at an end of the droplet

is reduced by applying a voltage to an electrode at that end of

the droplet. The electric field-induced reduction in interfacial

energy causes the droplet to locally spread out. The resulting

change in contact angles sets up a pressure gradient, which

drives the droplet toward the actuated electrode. Flow can

also be analyzed from energy-minimization considerations,

according to which the droplet minimizes its surface energy

by transiting to the actuated electrode. The energy gradient

is thus the driving force behind EW-induced motion of the

droplet.

The energy minimization based approach is first explained

through the prediction of capillary rise or fall due to applied

voltage to the capillary walls. Fig. 1 shows a circular capillary

of radius R and length L with a coating of a dielectric material

on its inner wall. The thickness of the dielectric layer is tand its dielectric constant is k. Application of voltage across

the dielectric layer changes the capillary height. The rise in

fluid level(capillary height) is analyzed by estimating the total

system energy as a function of the capillary height h. The total

energy is the sum of the dielectric-liquid interfacial energy, the

dielectric-air interfacial energy and the potential energy of the

liquid column. The system energy when the liquid meniscus

is at a height h with an applied voltage V is

E(h) = 2πRh

(γSL

0 − kε0V2

2t

)+2πR(L−h)γSA

0+πR2ρlgh

2

2(1)

where γSL0 , γSA

0 are the Dielectric-Droplet and

Dielectric-Air interfacial energy respectively. ε0 is the permit-

tivity of vacuum, g is acceleration due to gravity, ρl is droplet

density, k is the dielectric constant.

Differentiating the above equation with respect to h, we

obtain

∂E

∂h= 2πR

(γSL

0 − kε0V2

2t− γSA

0

)+ πR2ρlgh (2)

Obtained above, is the expression of force (i.e. rate of

change of momentum), which when further simplified can be

written as,

d(mυ)

dτ= C1 + C2h (3)

where m is the mass and υ is the velocity of droplet .

Or,

υ = C1(τ) + C2(τ)h (4)

where C1 and C2 are constant terms

The model in (4) can be applied to the droplet actuation

on a Lab-on-chip device. The meniscus height is analogues

to the length of electrode under the droplet (Fig.2) and gis analogues to horizontal acceleration of the droplet. The

electrodes are switched on and off periodically, for droplet

to move. As shown in Fig.2, when the second electrode is

switched on, surface energy of the droplet over the activated

electrode decreases. As a result droplet moves under energy

gradient towards right to attain the minimum energy configu-

ration. Similarly, the next electrode is switched on and second

electrode is switched off, so the droplet keeps on moving under

the energy gradient. According to (4), more is the value of h;

more is the velocity of the droplet.

Fig. 1. Schematic of Electrocapillary rise model

Fig. 2. Electrowetting actuator cross-section. The black (centre) elctrode is‘on’, and the white electrodes are ‘off’

26

III. SIMULATION

A channel of length 1200 μm is considered with a constant

pitch of 200 μm. Substrate used is SILICON 100, on which

a dielectric material glass is used, with its k equal to 2.

Droplet liquid is taken as water with its viscosity 5.01e-09

Mpa.s, density 9.9982e-016 kg/μm3, and k value of 1000.

The simulations for this channel configuration are done on

Coventorware R©. A process model was designed and a 3D

model of the channel is generated and meshed (Fig.3). While

keeping the pitch constant, the ratio of electrode length to gap

is varied and change in droplet velocity due to this variation

is observed. The ratio of electrode length to gap length(pitch

ratio) between two adjacent electrodes simulated are 1:3, 3:5,

1:1, 11:9, 5:3, and 3:1.The results of the simulation are shown

in Table I. Voltage used on all the electrodes are 60 V.

Frequency of switching electrodes used is 1670 Hz [10].

Fig. 3. 3D model of the EWOD based microchannel

The snapshot of the simulation result done on

Coventorware R© for the pitch configuration 3:1 ( Fig.

4). In Fig. 4(a), the droplet shown is at the beginning of

the simulation. As soon as voltage is applied to the second

electrode, the droplet distorts under the action of the surface

tension forces and change in the droplet-dielectric contact

angle (Fig. 4(b)). The droplet moves towards the right end

of the channel (Fig .4(c)) and finally reaches the right end as

shown in Fig. 4(d).

IV. RESULTS

The maximum velocity of the liquid droplet was observed

for the electrode to gap ratio of 3:1( length of electrode =

150μm, length of gap = 50 μm). As shown in Table I, as

we increase the length of the electrode, keeping the total

pitch length constant at 200μm, the velocity of the droplet

also increases. Increase in velocity of droplet with increase in

electrode length is also obtained from (4).

V. CONCLUSIONS

The results we obtained after simulation are found to be

in agreement with the proposed theory, and in both cases,

the velocity of the droplet increases with increase in pitch

ratio. The experimental values of droplet velocity against pitch

TABLE IVELOCITY READINGS FOR DIFFERENT PITCH RATIOS

Pitch Ratio Velocity of Droplet(cm/s)

1:3 6.2

3:5 9.2

1:1 13.6

11:9 16.7

5:3 20.4

3:1 50.1

Fig. 4. Snapshot of the simulated microchannel

as simulated in Coventorware R©, and the values obtained

from (4) are plotted in Fig. 5. It is observed that the velocity

distribution with pitch ratio in (4) is linear, whereas that

obtained using Coventorware R© is non-linear, but both show

the trend of increase in velocity with increase in pitch ratio. At

very low and high values of the pitch ratio, the deviation of the

experimental values from the values obtained from (4) is less,

while for the intermediate ratios, large deviation is observed.

The linear distribution of velocity with h in (4) is debatable

and needs to be rectified as the adopted model in (4) do not

take into account the change in shape of the droplet, which

is significant in case of LOCs. For low values of pitch ratio,

the deformation of the droplet is low, as lesser part of it is

over the electrode, and so the deviation is low. For very high

values of pitch ratio, deformation of droplet takes place, but

it happens for a very short time, as the droplet experiences

very large force. As a result, the effect of change of shape

on velocity is negligible, and so the deviation is low for high

values of pitch ratio. So, the velocity values obtained from (4)

and from Coventorware R© show very low deviation for smaller

27

and higher values of pitch ratios, while the deviation is large

in the intermediate range, as shown in Fig. 5. The refinement

of model for inclusion of non-linearity due to non-uniform

shape of the droplet is being worked further.

Fig. 5. Average droplet linear velocity versus pitch ratio for both simulationand theoretical values

ACKNOWLEDGMENT

We would like to acknowledge Aeronautical Development

Agency (ADA), India for providing the MEMS Design Center

with software facility under the aegis of NPMASS (National

Program for MEMS and Smart Sensors)

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